Prostaglandin synthetase is capable of metabolizing a number of substances previously thought to be metabolized only by the mixed-function oxidase system. A method has been developed for testing the ability of prostaglandin synthetase to metabolise xenobiotics to products mutagenic to Salmonella. Benzo(a)pyrene, benzanthracene, chrysene and a number of their metabolites have been tested for mutagenicity after activation by prostaglandin synthetase. A number of aromatic amines and related compounds including the bladder carcinogen benzidine, nitrosamines, and the phthalate derivative DEHP, have also been tested.